The deposition of thin films of aluminum nitride (AlN) has generated tremendous interest due to the unique combination of material properties that make AlN an important III-V compound. The large direct band gap of 6.2 eV, high melting point of 3273 K, high thermal conductivity of 320 W/mK at 300 K, thermal expansion coefficient match to GaAs (6.3.times.10.sup.-6 /K for AlN and 6.9.times.10.sup.-6 /K for GaAs), high acoustic velocity of 6 km/s, and low dielectric loss are excellent properties that make AlN the material of choice for many applications. Thin films of AlN are valuable for many applications including a gate dielectric and passivation layer in integrated circuits, optical coatings, heat sinks in electronic packaging applications, a material for surface acoustic wave devices, and wear-resistant, hard coatings for tribological applications. While several different methods are being employed for the deposition of AlN on various substrates, these methods typically involve heating the substrates to temperatures greater than 473 K. For a number of applications, most importantly in microelectronic devices involving GaAs and Si substrates, and also in tribological applications, the high temperatures needed to grow high-quality AlN films by currently available methods pose a serious problem due to the thermal sensitivity of the substrates. This problem is enhanced for GaAs devices, which suffer from lack of a suitable dielectric layer such as GaN, due to the inherent difficulties associated with growing GaN at low temperatures. Consequently, the growth of AlN films at low temperatures has become increasingly important and valuable.
The various methods that have been used to deposit AlN thin film include reactive sputter deposition, metalorganic chemical vapor deposition (MOCVD), chemical vapor deposition (CVD), microwave plasma CVD, electron cyclotron resonance metalorganic molecular beam epitaxy (ECR-MOMBE), laser chemical vapor deposition (LCVD),and pulsed laser deposition (PLD).
Reactive sputtering (rf or dc planar magnetron sputtering) has been used at substrate temperatures ranging from 473 K to 1473 K. The target is typically aluminum (Al) which is sputtered in the presence of nitrogen gas. One of the main problems associated with this technique is that in general the AlN films tend to be a multiphase material including both metallic Al and AlN. This mixture of phases makes it difficult to control the properties of these films. Electrical conductivity is strongly affected by Al filaments, inducing bistable resistance.
Several of the MOCVD and CVD techniques that have been aimed at epitaxial growth typically require high temperatures of 1200-1400 K. Epitaxial films of AlN on Si have been grown at 673 K using metalorganic surface chemical adsorption deposition (MOSCAD) with trimethylaluminum and ammonia as precursors. Chemical vapor deposition was used to produce amorphous films of AlN on Si at a temperature of 473-523 K. There are no known CVD techniques that have successfully deposited AlN films at temperatures lower than 473 K.
Microwave plasma CVD has been used to grow polycrystalline AlN on Si(100) substrates. The growth temperature was 823 K. The technique of ECR-MOMBE is both complex and expensive as it combines an expensive ultra-high vacuum MBE system with a custom designed ECR plasma source to generate nitrogen atoms in-situ. In addition, typical substrate temperatures were 823-1023 K.
LCVD methods for the growth of AlN and aluminum oxynitride (AlO.sub.x N.sub.y) have used trimethylaluminum and ammonia as precursors of aluminum and nitrogen respectively and 193 nm excimer laser radiation for photolysis. At substrate temperatures of 373-723 K, the films were primarily AlO.sub.x N.sub.y. AlN films were obtained only at temperatures greater than 723 K. Recently, LCVD has produced AlN films with good optical and electrical properties using trimethylaluminum and ammonia at temperatures of 443-473 K and 193 nm excimer laser radiation for photolysis. AlN films have also been grown on sapphire and GaAs substrates using both a xenon lamp and an excimer laser. These depositions were performed at fairly high substrate temperatures of 873-973 K.
PLD methods have used a KrF laser at 248 nm to deposit AlN films on sapphire at 943 K. Investigation of the morphological properties of the films indicated that they were poor in quality and contained numerous particulates. The deposition of AlN films has also been achieved using a pulsed ruby laser for evaporation of Al targets at 300 K. This method of deposition does not rely on gas phase laser photolysis. In addition, a ruby laser has poor output energies and is not the laser of choice for manufacturing processes. U.S. Pat. Nos. 5,356,608 and 5,221,527 also teach high temperature AlN deposition methods.
Most of the methods listed above, although successful in depositing AlN films, do so at substrate temperatures that are 473 K or higher. For microelectronic applications, it is desirable to lay down dielectric caps on pre-fabricated devices and wafers at low temperatures, in order to reduce thermal damage to the underlying devices. Specifically, for GaAs devices, it is extremely useful to be able to deposit films at as low a temperature as possible preferably lower than 473 K due to the added precautions that need to be taken to prevent the out-diffusion of As and also to prevent thermal degradation of the commonly used gold-germanium (Au/Ge) ohmic contacts. Additionally, for tribological applications, wear-resistant coatings are deposited typically on mechanical components that are made of stainless steel, which is susceptible to thermal degradation.
A study of the initial stages of AlN thin film growth by low-temperature chemical vapor deposition on alumina (Al.sub.2 O.sub.3) substrates has used trimethylamine alane (TMAA) N(CH.sub.3).sub.3 AlH.sub.3 and ammonia NH.sub.3 as precursors. Analytical techniques such as Fourier Transform Infrared spectroscopy and X-ray Photoelectron Spectrosopy (XPS) were used to monitor the reaction between the two precursors, following exposure of the substrate to high doses of both. Using XPS analysis, the authors claimed that the Al--N bond could be detected on the Al2O.sub.3 substrate at 400 K, but not at 300 K. From this it was inferred that a spontaneous chemical reaction between TMAA and NH.sub.3 on an alumina surface occurs at 400 K, but not at 300 K. However, no macroscopic and measurable thin film of AlN was deposited in this study.
All of the prior methods have not provided good quality AlN at a low temperature. These and other disadvantages are solved or reduced using the present invention.